Astrophysics > Cosmology and Nongalactic Astrophysics

Title:
The Isotropic Radio Background and Annihilating Dark Matter

Abstract: Observations by ARCADE-2 and other telescopes sensitive to low frequency
radiation have revealed the presence of an isotropic radio background with a
hard spectral index. The intensity of this observed background is found to
exceed the flux predicted from astrophysical sources by a factor of
approximately 5-6. In this article, we consider the possibility that
annihilating dark matter particles provide the primary contribution to the
observed isotropic radio background through the emission of synchrotron
radiation from electron and positron annihilation products. For reasonable
estimates of the magnetic fields present in clusters and galaxies, we find that
dark matter could potentially account for the observed radio excess, but only
if it annihilates mostly to electrons and/or muons, and only if it possesses a
mass in the range of approximately 5-50 GeV. For such models, the annihilation
cross section required to normalize the synchrotron signal to the observed
excess is sigma v ~ (0.4-30) x 10^-26 cm^3/s, similar to the value predicted
for a simple thermal relic (sigma v ~ 3 x 10^-26 cm^3/s). We find that in any
scenario in which dark matter annihilations are responsible for the observed
excess radio emission, a significant fraction of the isotropic gamma ray
background observed by Fermi must result from dark matter as well.